1. Field of the Invention
The present invention relates to a control method of an AC synchronous motor, in particular, to a phase lead control method.
2. Description of the Related Art
In controlling an AC synchronous motor, as the revolving speed of the motor increases to enter high speed range, the power factor of the motor tends to fall due to the phase delay of the supplied current. In order to prevent the fall of the power factor, a phase advancing compensation control, which controls the phase of a current command by advancing the phase, is generally employed.
FIG. 1 is a block diagram showing a current control system for executing the phase advancing compensation control. In FIG. 1, a phase of a current command (torque command) Tcmd is advanced by .delta. using a phase advancing module 1, and the current command T, whose phase is advanced, is treated as a current command for a current loop. According to current loop processing, the difference between the current command T, whose phase is advanced, and an actual current I of a motor is calculated, and the calculated difference is integrated by an integrating module 2, and is further multiplied by an integrating constant k1. A proportional module 3 multiplies the actual current I by a proportional constant k2. An output value of the proportional module 3 is subtracted from an output value of the integrating module 2, and a counter electromotive force correction value kEo is added to the difference. The value thus. obtained is treated as a command voltage (terminal voltage) Vc supplied to the motor.
A module 4 is a term representing a motor coil. A voltage actually applied to the motor coil (R and L represent resistance and inductance, respectively) is a voltage obtained by subtracting a counter electromotive force Eo from the aforesaid command voltage (terminal voltage) Vc, and a current I flows.
According to the aforesaid phase advancing module 1, the phase advancing compensation amount .delta. is calculated by a linear function proportional to an absolute value of the motor's revolutionary speed v, and is ordinarily obtained by a function expressed by the following equation (1): EQU .delta.=K.multidot..vertline.v.vertline. (1)
Where, K is a proportional constant.
Using the phase advancing compensation amount .delta. and based on the position of the rotor, i.e., based on the counter electromotive force Eo and the current command Tcmd, the phase-advanced current command T (a vector) can be expressed as follows: EQU T=[To.multidot.cos .delta.To.multidot.sin .delta.].sup.T ( 2)
Where Tcmd=[To 0].sup.T
The actual current I (vector) is given as follows: EQU I=[x1.multidot.To y1.multidot.To].sup.T ( 3)
In addition, the following equation (4) is obtained on the basis of the relationship between the command voltage applied to the motor coil and the voltage applied to the coil side as is shown in FIG. 1. EQU (k1/j.omega.)(T-I)-k2.multidot.I+k.multidot.Eo=Eo+(R+j.omega.L)I(4)
If a current vector [x1 y1].sup.T is calculated by the above equation (4), the following equation (5) is obtained. ##EQU1##
FIGS. 3 through 6 are vector diagrams drawn on the basis of the above equations (4) and (5). FIGS. 3 and 4 show vector diagrams in the case where the motor is accelerated at a velocity of 2000 rpm. FIG. 3 shows a vector diagram in the case where the phase advancing compensation amount .delta. is set to "0" (zero). FIG. 4 is a vector diagram in the case where the phase advancing compensation .delta. is set to 99.7 degrees. In FIG. 3, the actual current I is delayed by .theta.1 with respect to the current command Tcmd (counter electromotive force Eo); on the other hand, the command voltage Vc becomes more than a DC link voltage which is a power-supply voltage of an inverter. Therefore, the power factor becomes poor. In FIG. 4, the command voltage Vc is within the DC link voltage, and power factor becomes better. In other words, the power factor is improved by the phase advancing control.
FIGS. 5 and 6 are vector diagrams in the case where the motor is decelerated at a velocity of 2000 rpm. FIG. 5 shows a vector diagram in the case where the phase advancing compensation amount .delta. is set to "0" (zero). FIG. 6 shows a vector diagram where the phase advancing compensation amount .delta. is set to 99.7 degrees. As is obvious from the comparison between FIGS. 5 and 6, when the phase advancing control is executed, the command voltage Vc becomes higher than the DC link voltage which is a power-source voltage of the inverter for controlling the motor. This means that a voltage cannot be applied up to the command voltage Vc. In other words, if the phase advancing correction is equal to that in the case of the acceleration, the phase correction becomes excessive.